(i) Field of the Invention:
This invention relates to a cathode for an electron source which is useful in electron beam-applying equipment such as electron microscope and electron microfabrication system, and a method of producing the cathode.
(ii) Brief Description of the Prior Art:
Carbides of elements of groups IV, V and VI in the periodic table and silicon, or borides of the alkaline earth and the rare earth are excellent electron emissive materials. Especially in the field of scientific instruments applying electron beams, they are replacing conventional cathodes employing tungsten. Lanthanum hexaboride (LaB.sub.6), for example, has come into use for the electron source of a scanning electron microscope or an electron microfabrication system as a thermionic cathode material which exhibits a brightness higher than that of tungsten. Since high-melting carbides such as titanium carbide (TiC) and silicon carbide (SiC) interact little with residual gases in a vacuum and are immune to ion bombardment, they are noticed as materials for field emission cathodes of high stability and long life.
The cathodes of the thermal emission (hereinafter, abbreviated to "TE") type and the field emission (hereinbelow, abbreviated to "FE") type need to be heated to a high temperature during operation or to be heated to a high temperature in order to clean the cathode surface prior to operation. As methods of heating, there are the indirect heating method which exploits electron bombardment or the like, and the conduction heating method in which a conductive support or filament for holding the cathode is supplied with electric power so as to directly heat the cathode. With the indirect heating method, the structure of the cathode becomes complicated, so that the heat loss is usually heavy and that a high power is required for heating the cathode. On the other hand, in case of the conduction heating method, the structure of the cathode is simple and the electric power required for the heating may be low, so that it is a desirable heating method for the cathode. Especially in case of the FE cathode, it is ordinarily necessary to elevate the temperature to above 2,000.degree. C., and a structure capable of conduction heating is required in order to effectively heat the cathode. Also in case of the TE cathode, the conduction heating is desirable as stated above.
The cathode capable of conduction heating is generally made up of a structure in which an electron emissive material is spot-welded to the central part of a conductive support made of a high-melting metal wire and according to which the cathode can be heated to a desired temperature by causing current to flow through the conductive support. A material for the conductive support needs to be one which is difficult to react with the electron emissive material. As high-melting conductive materials difficult to react with carbides and borides, the same sorts of carbides and borides and besides carbon are known. Since, however, carbides and borides are high in the cost of raw materials and are difficult in the working, they are undesirable as practical filament materials. In case of employing carbon as the filament material, there is no appropriate method for attaching the carbide or boride of the electron emissive material to the carbon filament, and hence, various contrivances are made for the attachment. By way of example, in case of LaB.sub.6 being the TE cathode material, there have been proposed an expedient wherein the LaB.sub.6 cathode is sandwiched between two bars of pyrolytic graphite and thus mechanically pressed and secured, and an expedient wherein a hole is provided at the center of a square pillar of LaB.sub.6, two graphite sheets placed one over the other are passed therethrough, and spacers are fitted on end parts of the two graphite sheets thereby to bestow a spring action. Due to the complicated cathode structures, however, these methods involve problems in the aspects of handling, stability, reproducibility etc. and inevitably render the operating life short. Moreover, inasmuch as these measures do not perform the conduction heating very effectively, they cannot be applied to the FE cathode which is heated to a high temperature above 2,000.degree. C. No favorable result has been obtained even in the TE cathode.
As regards the FE cathode of TiC, there has been reported an expedient wherein TiC is bound with a high-melting metal wire such as Ta wire, and this portion is fixed by being coated with a raw thermosetting resin such as phenol resin and then subjected to carbonization. However, in case where the cathode is repeatedly heated to the high temperature, such problems take place that the high-melting metal wire is carbonized to become fragile and that the portion with TiC fixed comes off on account of the differences among the coefficients of thermal expansion of TiC, the high-melting metal wire and carbon. It is therefore hard to say that this method of fixation is satisfactory in practical use.
Since carbides and borides are fragile and cannot be subjected to the spot welding, it is the actual situation that a structure of a cathode capable of simple and effective conduction heating as comparable to the tungsten cathode and a method of producing the same have not been established yet. This has been a serious obstacle to putting into practical use carbides and borides which are excellent electron emissive materials.
Techniques close to this invention are described in "S. F. Vogel; The Review of Scientific Instruments," vol. 41, No. 4 (April 1970), pages 585-587 and Japanese Unexamined Published Patent Applications No. 52-22468 and No. 51-55666. U.S. Pat. No. 4,054,946 teaches an invention of an antecedent application in U.S. although it was not publicly known prior to the original Japanese patent application of this invention.